Can Solar Cope with UK Energy Demand Today? and in the Future?

Artem Shargorodskii

d’Overbroeck’s College 2017

Contents

Our future with fossil fuels 2

Can we prevent a catastrophe? 2

Current condition of solar industry 4

Why has solar industry started to expand quite recently? 6

Is it sensible to supply the UK solely with solar power today? 8

How to reduce the area of the solar plant? 11

Efficiency of solar panels today 13

How do solar panels work? 14

Factors limiting solar cell efficiency 15

What is the maximum theoretical efficiency of a solar cell? 17

What will be the efficiency of solar panels in the future? 17

What area of the solar plant can we expect in the future? 19

Is this project realistic? 20

Ways to make the project more realistic 21

Conclusion 22

Appendix: Glossary 23

Bibliography 24

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Our future with fossil fuels

How soon are we going to run out of fossil fuels? This question has been raised a lot of times in the recent years. In May 2014, BBC reported that in just over 5 years Britain will have run out of oil, coal and gas. Researchers have calculated that Britain has just 5.2 years of oil, 4.5 years of coal and 3 years of its own gas remaining. Does it mean that in 2019 Britain will face an 1

apocalypse? No, because new reserves will be discovered. Reserves are fossil fuels that are known to be recoverable. From this moment we would use up all our reserves in 5 years, 2

however, we would not run out of resources of fossil fuels. Resources are the often much larger amounts known or suspected to be available. They might not be recoverable due to technical 3

challenges or high costs. Firms which extract fossil fuels continually explore resources to find more reserves. However, new reserves of fossil fuels are becoming harder to find, and those that are being discovered are significantly smaller than the previous ones. In addition, energy 4

demand of the UK keeps increasing as population grows.

At some point the UK will not be able to produce enough energy to supply its own demand. Imported energy might safe situation for some time, however, falling supply and rising demand for fossil fuels would result to extremely high prices globally. Energy bills in the UK will rise massively while countries like Venezuela, Saudi Arabia, Russia and USA will be able to 5

manipulate the world politically and economically due to their large fossil reserves. This will continue until all countries will run out of fossil fuels completely and there will be no way back. Can we prevent a catastrophe?

There is no doubt that we will run out of fossil fuels at some point. But instead of panicking and trying to find out when will it happen we simply need to substitute scarce fossil fuels for renewable sources like the Sun, wind, geothermal heat, river flow, sea waves etc. Renewables are entirely free to exploit once the infrastructure is built, they are available to everyone and most importantly they will never run out. Renewable energy sources offer us another way, a way to avoid a potential catastrophe. But which of the renewable sources has the greatest potential? What is the energy source of the future?

‘UK needs more home-grown energy', BBC News, 1

<h?p://www.bbc.co.uk/news/science-environment-27435624> [accessed 3 August 2017]

Simon Evans, ‘Factcheck: Why the UK will not run out of oil, coal or gas in five years’, Carbon Brief, 2

<h?ps://www.carbonbrief.org/factcheck-why-the-uk-will-not-run-out-of-oil-coal-or-gas-in-five-years> [accessed 3 August 2017]

Ibid.3

‘The end of fossil fuels’, Ecotricity, 4

<h?ps://www.ecotricity.co.uk/our-green-energy/energy-independence/the-end-of-fossil-fuels> [accessed 5 August 2017]

‘List of countries by proven oil reserves’, Wikipedia, 5

<h?ps://en.wikipedia.org/wiki/List_of_countries_by_proven_oil_reserves> [accessed 5 September 2017]

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Figure 1 – Global potential capacity by source 6

If you look at global potential capacity of each source, annual solar power reserves look tremendous compared to other sources. Note that those are spheres - if they were circles, wind would have the diameter of a ping pong ball, coal would be like a bowling ball, and solar like a small house. Every year the Sun produces more energy than all other renewable sources 7

together with energy equivalent of all reserves of fossil fuels on our planet. Elon Musk, a co-founder of SolarCity, calls the Sun a “handy fusion reactor in the sky called the Sun. It just works. It shows up every day and produces ridiculous amounts of power.” In fact, the Sun 8

radiates more energy to the Earth in a couple hours than all of humanity consumes from all sources annually. Clearly, solar has a huge potential of being energy source number one in the 9

future.

Every source of renewable energy produces little or no waste products such as carbon dioxide or other chemical pollutants, so has minimal impact on the environment. However, solar is the 10

most perspective and, as a result, the fastest growing renewable energy source because it has several advantages over other environmentally friendly alternatives. Solar panels do not need as much maintenance as wind turbines and hydroelectric dams do. Unlike biofuel plants and wind turbines, sun power does not create pollution or noise. In contrast to tidal and geothermal 11

power, solar does not have a reliance on being near specific locations like coast or tectonic plates.

In addition, solar dominance over other renewables can be explained by the fact that solar panels can be easily deployed by both home and business users. Barriers to start generating 12

solar energy are very low since solar panels do not require any huge set up like in case of other

Tim Urban, ‘The Deal with Solar’, Wait But Why, 6

<h?ps://waitbutwhy.com/2015/06/the-deal-with-solar.html> [accessed 21 June 2017]

Tim Urban, ‘The Deal with Solar’, Wait But Why7

Cramine Gallo, ‘Tesla's Elon Musk Lights Up Social Media With A TED Style Keynote’, Forbes, 8

<h?ps://www.forbes.com/sites/carminegallo/2015/05/04/teslas-elon-musk-lights-up-social-media-with-a-ted-style-keynote/#5dd1a0d57ad2> [accessed 8 September 2017]

Tim Urban, ‘The Deal with Solar’, Wait But Why9

‘The Advantages and Disadvantages of Renewable Energy’, Solarschools.net, 10

<h?p://www.solarschools.net/resources/stuff/advantages_and_disadvantages.aspx> [accessed 11 September 2017]

‘Advantages of Solar Power Over Alternate Energy Sources’, Soluxe, 11

<h?p://www.soluxesavings.com/advantages-solar-power-over-alternate-energy-sources-a-122.html> [accessed 11 September 2017]

Rinkesh, ‘What is Solar Energy?’, Conserve Energy Future, 12

<h?ps://www.conserve-energy-future.com/advantages_solarenergy.php#abh_posts> [accessed 21 September 2017]

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renewables. Once panels are installed, they require little maintenance due to their high durability so almost no additional costs are incurred. This enables solar energy generation industry to have a lot of suppliers, therefore, there is no chance of monopoly or oligopoly formation. In contrast, oil industry has a dominant cartel called OPEC (Organization of the Petroleum Exporting Countries) which consists of 12 of the world's major oil-exporting nations such as Venezuela, Saudi Arabia, Iran, Iraq and others. OPEC is accounted for an estimated 44% of global oil production and 73% of the world's proven oil reserves, giving it a major influence on global oil prices. It would be impossible to create a cartel such as OPEC in solar 13

industry due to low barriers for entry. When the price for solar energy is high or subsidized by the government, a lot of new suppliers join the market lowering the price of energy which is beneficial for consumers. Conversely, oil industry has very high entry barriers, so OPEC can feel safe with no risk of facing competition.

‘OPEC’, Wikipedia, 13

<h?ps://en.wikipedia.org/wiki/OPEC> [accessed 5 September 2017]

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Current condition of solar industry We have already convinced that solar power beats all fossil fuels in potential reserves, environmental friendliness and ease of setting up. Logically, this should result to a significant share of solar power in total energy production, however, the reality is far from this being true.

Figure 2 (left) – Global primary energy consumption by source 14

Figure 3 (right) – UK primary energy consumption by source 15

Globally, only 0.45% of primary energy and 1.3% of all the electricity 16 17

demand is produced by solar power. In the UK, 10,292 GWh of 18

electricity is produced by photovoltaics systems which makes up 0.46% of 2,242,520 GWh (192,822 ktoe ) of total primary energy consumption. 19

Clearly, neither solar nor the entire renewable energy sector is even close to be the main source of energy in the world or in the UK. However, such a miserable relative size of solar energy generation can be explained by the fact that solar power expansion has started to occur quite recently.

Hannah Ritchie and Max Roser, ‘Energy Producion & Changing Energy Sources’, Our World in Data, 14

<h?ps://ourworldindata.org/energy-producion-and-changing-energy-sources/> [accessed 5 July 2017]

Simon Evans, ‘Five charts show the historic shiks in UK energy last year’, Carbon Brief, 15

<h?ps://www.carbonbrief.org/five-charts-show-the-historic-shiks-in-uk-energy-last-year> [accessed 3 October 2017]

‘World Energy Resources 2016’, p. 4, World Energy Council, 16

<h?ps://www.worldenergy.org/wp-content/uploads/2016/10/World-Energy-Resources-Full-report-2016.10.03.pdf> [accessed 17 September 2017]

‘World Energy Resources 2016’, chap. Solar, p. 23, World Energy Council17

‘Energy Trends March 2017’, p. 69, Department of Business, Energy & Industrial Strategy, 18

<h?ps://www.gov.uk/government/uploads/system/uploads/a?achment_data/file/612492/Energy_Trends_March_2017.pdf> [accessed 5 August 2017]

‘Energy Consumpion in the UK’, p. 9, Department of Business, Energy & Industrial Strategy, 19

<h?ps://www.gov.uk/government/uploads/system/uploads/a?achment_data/file/633503/ECUK_2017.pdf > [accessed 4 August 2017]

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10,292 GWh / 2,242,520 * 100% = 0.46%

Figure 4 – Comparative global primary energy consumption 20

Initially, it could seem that there has been no change in the distribution of global primary energy over the last 10 years. However, due to small size of solar power share it is hard to notice its tremendous growth. If you look at the percentage share of solar power (dark orange) you will notice that in 2005 only 0.01% of total energy was produced by solar. In 2010, already 0.06% - 6 times more, and in 2015, relatively massive 0.45% - 7.5 times more than 5 years before and 45 times more than in 2005. Such a rapid growth in global solar energy production can be explained by an exponential growth of global installed capacity of solar PV modules.

Figure 5 – Global cumulative photovoltaic capacity over time 21

Clearly, solar power capacity has been growing exponentially over the last 17 years. Overall, solar capacity grew more than 10 times from 2009 to 2015, and more than 100 times from 2002 to 2015 . In 2017 global solar energy capacity reached 401,500 GW , 1.5 times more than it 22 23

was in 2015 resulting to the continuation of stable exponential growth pattern.

‘World Energy Resources 2016’, p. 4, World Energy Council20

Zachary Shahan, ‘10 Solar Energy Facts & Charts You (& Everyone) Should Know’, Clean Technica, 21

<h?ps://cleantechnica.com/2016/08/17/10-solar-energy-facts-charts-everyone-know/> [accessed 13 September 2017]

Ibid.22

‘Growth of photovoltaics’, Wikipedia, 23

<h?ps://en.wikipedia.org/wiki/Growth_of_photovoltaics> [accessed 13 September 2017]

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Figure 6 – UK cumulative photovoltaic capacity over time 24

In the UK, solar power expansion took place in 2011 when it started to gain momentum rapidly. British solar capacity grew 10 times from 2011 to 2015, more than 100 times from 2010 to 2015 and more than 1000 times from 2004 to 2015. In 2016 PV capacity reached 11,630 MW 25 26

resulting to a 30% growth over the previous year. High pace of growth indicates that the industry has just started to expand so every additional PV power plant results to a high percentage growth of the total capacity. However, is the fast growth of PV capacity going to continue in the future and what does determine its pace? Why has solar industry started to expand quite recently?

Figure 7 – Global investment in renewable energy by technology 27

The main reason for such a rapid growth of solar PV capacity is a growing investment in the renewable energy production. In 2004, the world invested $47 billion in renewables and by 2015, this had increased to $286 billion, an increase of 500%. Over the last 10 years 28

investment in PV technology has doubled as a percentage of total investment in renewables. In 2016, solar and wind energy received 47% of total investment each resulting to a combined share of 94%. 29

0

2250

4500

6750

9000

2000 2002 2004 2006 2008 2010 2012 2014

Capacity (MW)

Ibid.24

Ibid.25

Ibid.26

Hannah Ritchie and Max Roser, ‘Energy Producion & Changing Energy Sources’, Our World in Data27

Ibid.28

Ibid.29

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Figure 8 – UK investment in renewable energy by technology 30

In 2015, a record £15.2 billion was invested in UK clean energy. Currently, the UK focuses 31

more on wind power – 65% of total investment in renewables accounts for wind power, however about 20% of money goes to solar industry taking the second largest share of total investment after wind.

In the recent years, investment into renewables, especially solar and wind technologies, has been surging all over the world. But why now? Why not 10 years or 20 years ago? Is it because the urgency of switching to renewables is now great enough since the prospect of running out of fossil fuels is very close? This might also be a cause, however our fossil fuel reserves are still 32

not critically endangered so there should be another contributing factor.

In 2015, Michael Liebreich, chairman of the advisory board at Bloomberg New Energy Finance commented on the figures of the record high investment in renewables that year: “These figures are a stunning riposte to all those who expected clean energy investment to stall on falling oil and gas prices. They highlight the improving cost-competitiveness of solar and wind power.” 33

Exactly! The reason for the renewables boom happening right now is that their prices are now comparable with prices of fossil fuels.

Simon Evans, ‘Analysis: Record UK renewable energy investment overtakes North Sea spend’, Carbon Brief, 30

<h?ps://www.carbonbrief.org/analysis-record-uk-renewable-energy-investment-overtakes-north-sea-spend> [accessed 18 September 2017]

Ibid.31

Kathryn Senior PhD, ‘When Will Fossil Fuels Run Out?’, Carbon Counted, 32

<h?p://www.carboncounted.co.uk/when-will-fossil-fuels-run-out.html> [accessed 5 September 2017]

Nina Chestney, ‘Global clean energy investment hits record $329 billion in 2015’, Reuters, 33

<h?ps://www.reuters.com/aricle/uk-global-renewables-investment/global-clean-energy-investment-hits-record-329-billion-in-2015-idUKKCN0UT0Z6> [accessed 18 October 2017]

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Figure 9 – Price of energy in the US by source 34

The drop in costs of solar PV has been tremendous: since 1980 the cost fell in 20 times. However, it is not as important as the fact that the tip of the curve representing solar energy costs came very closely to fossil fuels costs. In 2013, PV cost passed the level of electricity retail prices in the US (they differ in other countries) and was very close to the price of crude oil and natural coal. This means that in the recent years solar power has become very competitive with fossils in terms of price. An advantage of solar power will only grow in the future. Solar PV modules will be developed further due to large investments, so their efficiency and productivity will improve while the price for a given output will reduce. On the other hand, cost of fossil fuels will start to go up due to their scarcity and reducing investment. Thus, in the future it will be unprofitable and insensible to build a power plant working on fossil fuels rather than on solar PV modules.

‘The Beginning of the End?’, Architecture 2030, 34

<h?p://architecture2030.org/the-beginning-of-the-end-of-the-fossil-fuel-era/> [accessed 12 October 2017]

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Is it sensible to supply the UK solely with solar power today? Photovoltaic panels can transfer electromagnetic (light) energy into electrical. Therefore, if we want to supply the UK purely with solar energy we need to make sure that everything runs up solely on electricity starting from transport and factories and finishing with heating systems and stoves. In 2016, total final energy consumption in the UK was 1,636 TWh (140,668 ktoe ) of 35

which 304 TWh was consumed in the form of electricity. Therefore, nowadays only 18.6% of 36

all the energy we use annually accounts for electricity whereas our goal is 100%.

Complete electrification of the entire UK will not be an easy transition and will take a lot of time and effort, however, it will ensure us a safe future in terms of environment and, most importantly energy supply. Let’s imagine that electrification has already happened in the UK. In this case, how much electricity should a single solar plant be able to produce annually to supply the whole country with enough energy? To find the total output energy of a solar plant we need to consider final energy consumption and energy dissipated in the UK national grid. Currently, annual final energy consumption in the UK is 1,636 TWh and overall losses in the national grid add up to about 7.7%. Therefore, to meet annual energy demand of the UK a total 37

output of our solar plant should add up to 1,772.5 TWh. The next question is how much space would we need to allocate for a solar plant producing 1,772.5 TWh of electricity annually. To answer it, we can use the following formula:

E = A * r * GTI * PR 38

• E - total energy produced by a solar plant in one year (kWh) • A - total area of solar panels (m2) • r - efficiency of a solar panel. It is a measurement of which proportion of the available solar

energy a solar panel converts into electrical energy. 39

• GTI (Global Tilted Irradiation) - annual average radiation received by a surface of a tilted panel (kWh/m2). GTI normally varies between 200 kWh/m2 (Norway) and 2,600 kWh/m2 (Saudi Arabia). 40

• PR (Performance Ratio) is the ratio of energy which enters electric grid with respect to the output of a solar panel. It indicates which portion of the generated current actually reaches

‘Energy Consumpion in the UK’, p. 5, Department of Business, Energy & Industrial Strategy35

‘DUKES chapter 5: staisics on electricity from generaion through to sales’, p. 111, GOV-UK, 36

<h?ps://www.gov.uk/government/uploads/system/uploads/a?achment_data/file/633779/Chapter_5.pdf> [accessed 9 August 2017]

‘Naional Grid (Great Britain)’, Wikipedia, 37

<h?ps://en.wikipedia.org/wiki/Naional_Grid_(Great_Britain)#Losses> [accessed 23 October2017]

‘How to calculate the annual solar energy output of a photovoltaic system?’, Photovoltaic SoQware, 38

<h?p://photovoltaic-sokware.com/PV-solar-energy-calculaion.php> [accessed 24 October 2017]

Serm Murmson, ‘The Average Photovoltaic System Efficiency’, Sciencing, 39

<h?ps://sciencing.com/average-photovoltaic-system-efficiency-7092.html> [accessed 24 October 2017]

‘How to calculate the annual solar energy output of a photovoltaic system?’, Photovoltaic SoQware40

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1,636 TWh / (1 - 0.077) = 1,772.5 TWh

the grid after being affected by inverter losses, cable losses, temperature losses, etc. PR 41

usually ranges between 0.5 and 0.9 with a standard value of 0.75. 42

By rearranging this equation, we can get the expression of total area of solar panels (A) in terms of other variables:

A = E / (r * GTI * PR)

The annual output of the solar plant should be equal to the final energy consumption including power grid losses, therefore E = 1,772.5 TWh (1,772.5 * 109 kWh). Nowadays, most typical silicon solar cells have a maximum efficiency of around 15% , therefore r = 0.15. Performance 43

ratio of an average solar set up is about 0.75 , so PR = 0.75. The last factor we need to 44

consider is the annual solar irradiation which depends entirely on the location of a power plant. Therefore, we firstly need to choose a location for our solar plant to be able to calculate the area required for it.

Figure 10 (left) – GTI distribution map across the UK territory 45

Figure 11 (center) – British Isles population density map 46

Figure 12 (right) – British Isles terrain map 47

‘Performance Raio’, PVsyst, 41

<h?p://files.pvsyst.com/help/performance_raio.htm> [accessed 28 October 2017]

‘How to calculate the annual solar energy output of a photovoltaic system?’, Photovoltaic SoQware42

Serm Murmson, ‘The Average Photovoltaic System Efficiency’, Sciencing43

‘How to calculate the annual solar energy output of a photovoltaic system?’, Photovoltaic SoQware44

‘Global Tilted Irradiaion map’, Global Solar Atlas, 45

<h?p://globalsolaratlas.info/?c=56.734649,-2.713623,7&s=57.50402,-2.296143&m=sg:gi> [accessed 17 October 2017]

‘Briish Isles populaion density 2011’, Wikipedia, 46

<h?ps://commons.wikimedia.org/wiki/File:Briish_Isles_populaion_density_2011_NUTS3.svg> [accessed 18 October 2017]

Hallgeir Gjesdal, ‘Map colored by elevaion like UK topo map’, Locus Map, 47

<h?p://help.locusmap.eu/topic/hello-theme-builders> [accessed 18 October 2017]

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On the one hand, it is more sensible to install solar panels in the Southern part of the UK where solar irradiation is the highest. However, we should also consider population density and terrain of a location. It is easier to set up a solar plant in the areas with low population since there is more unallocated space which can be used for our purposes. In addition, installing solar panels on the flat ground surface is much cheaper, less labour intensive and quicker than doing it in the mountainous terrain. Unfortunately, it is rarely the case when a region is both flat and sparsely populated since people do not tend to live in the mountains. This means that in most cases we should choose between areas with flat surface but high population density or with mountainous terrain but low population density.

Realistically, nowadays it is very unlikely to come to an agreement with the government on the construction of a photovoltaic plant if it requires massive resettlement of citizens. This might change in the future as soon as transition to the renewable energy will become a top priority when the prospect of running out of fossil fuels will be critically close. But for now, it is not realistic to expect a governmental approve to build a massive photovoltaic power station in highly populated areas which are mostly concentrated in the Central and Southern England.

Figure 13 – United Kingdom map with marked position 48

An area marked with a red point is situated in the Northern Scotland and has a combination of a relatively flat terrain and low population density which is perfectly suitable for our purposes. At that point GTI is equal to 1,050 kWh/m2 per year, however the total area of the solar plant will 49

be larger than a specific point so in same parts of the plant the GTI might be different. However, in that region sun radiation is distributed quite uniformly so we can assume that an average GTI = 1,050 kWh/m2. Now we have enough data to calculate the total area of a solar power plant in Scotland which could supply the whole UK with electricity. Substituting all the known values into equation we get the area of 15,000 km2. But how big is it?

United Kingdom, Google Maps, 48

<h?ps://www.google.nl/maps/@54.4129609,-4.4466634,6z?hl=en> [accessed 4 August 2017]

‘Global Tilted Irradiaion map’, Global Solar Atlas49

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A = E / (r * GTI * PR) = 1,772.5 * 109 kWh / (0.15 * 1,050 kWh/m2 * 0.75) = 1.5 * 1010 m2 = 15,000 km2

Figure 13 – United Kingdom map with marked position and area 50

As you can see from the map, 15,000 km2 is an enormous area – 6.2% of the total UK area or almost three quarters 51

(72%) of Wales. Certainly, this project is not realistic 52

since the area taken under solar plant is too large relatively to the UK total area. It is actually underestimated since we have not considered module inter-row spacing. 15,000 km2 is the area of solar panels if they were installed tightly one to another so if one module somewhere in the middle of our solar plant is broken we would need to repair it from the helicopter, for example. In addition, when there is no space between the rows of PV modules the shadow from one row drops on the next one, so the total output of solar plant is reduced massively. The point is that in reality this area would be at least three times larger than what we have calculated. Clearly, even in the South of the UK where an average GTI is 1330 kWh/m2 per year , 26% higher than in Scotland, a solar plant 53

would still occupy a vast territory.

How to reduce the area of the solar plant?

Obviously, today nobody would start building a solar plant which could substitute all other energy sources, however, this does not mean that it will be impossible forever. Some factors are not amenable to our influence like annual solar irradiation while others can be possibly improved in the future like efficiency of PV modules. Our previously used formula can help us to identify the factors we need to improve to make the area of our PV plant more realistic.

A = E / (r * GTI * PR)

United Kingdom, Google Maps50

‘United Kingdom - Geography’, Wikipedia, 51

<h?ps://en.wikipedia.org/wiki/United_Kingdom#Geography> [accessed 14 September 2017]

Ibid.52

‘Global Tilted Irradiaion map’, Global Solar Atlas, 53

<h?p://globalsolaratlas.info/?c=51.239566,-0.785522,9&s=50.819818,-0.722351&m=sg:gi> [accessed 17 October 2017]

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15,000 km2 / 242,495 km2 * 100% = 6.2%

15,000 km2 / 20,779 km2 * 100% = 72%

Firstly, the area is directly proportional to the final energy consumed annually so if we could reduce our energy consumption we would need less solar panels.

Figure 14 – Primary energy consumption in the UK by source 54

It is hard to estimate if the energy consumption will be growing or falling in the future since the energy demand is very volatile due to two contradicting factors. On the one hand, energy demand increases as population of the UK grows. It happens because industry sector produces more output to provide it to the larger population and more people need gas and electricity for domestic purposes and fuel for transport.

Figure 15 – Final energy consumption in the UK by sector 55

On the other hand, as time passes new technologies are developed which improve the efficiency of energy generation and transportation so less primary energy is required to produce the same output. In addition, in the recent decades an economic activity has shifted away from heavy, energy intensive industries. 56

Another way to reduce the area of the PV plant is to find a location with higher annual solar irradiation. The maximum value of GTI within the British Isles can be obtained in the South of England, in the southern parts of counties like West Sussex, East Sussex, Kent and Isle of Wight.

Hannah Ritchie and Max Roser, ‘Energy Producion & Changing Energy Sources’, Our World in Data54

‘Energy Consumpion in the UK’, p. 7, Department of Business, Energy & Industrial Strategy,55

Ibid., p. 856

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Figure 16 – GTI distribution map across the South of England 57

An average GTI in the light green areas near the coast is 1330 kWh/m2 per year . If we want to 58

build a solar plant on the UK territory than 1330 kWh/m2 is the maximum annual irradiation available. The closer the PR value of a PV plant approaches 100%, the more efficiently the respective PV plant is operating, therefore the smaller the required area with a given output. In real life, a value of 100% cannot be achieved, as unavoidable losses always arise with the operation of the PV plant. Nowadays some power plants with solar modules based on crystalline cells, the 59

most popular technology nowadays, can reach a performance ratio of 0.85 to 0.95. To reach 60

such a high performance ratio, extremely efficient inverters, cables and other equipment must be used in addition to PV modules themselves. The surface of the panels should always be clear from dust, snow and shadow and PV modules should be cooled down properly to minimize thermal losses. If a solar plant with the most efficient equipment available for today can reach a performance ratio of 0.95, then in the future it will still be available but for lower price. Efficiency of solar panels today

The last but not the least variable we can change is the efficiency of a PV module. Efficiency is the fraction of energy in the sunlight striking the cells that is converted to electrical energy. The higher the efficiency, the fewer the cells required to produce a given amount of electricity, or the more electricity can be generated with the given number of cells. 61

‘Global Tilted Irradiaion map’, Global Solar Atlas57

Ibid.58

‘Performance raio’, SMA, 59

<h?p://files.sma.de/dl/7680/Perfraio-TI-en-11.pdf> [accessed 23 October 2017]

‘Performance Raio’, Solar Server, 60

<h?ps://www.solarserver.com/knowledge/lexicon/p/performance-raio.html> [accessed 23 October 2017]

Harold Schobert, Energy: the basics (Routledge, 11 December 2013), p. 17761

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Figure 17 – Solar panel efficiencies by manufacturer 62

Nowadays, an average photovoltaic panel has an efficiency of 15-17%. The most efficient commercially available solar panels on the market today have efficiency ratings as high as 22.5%. The most efficient solar cell ever created can convert up to 46% of sun energy into 63 64

electricity. The question is what does stop us from building a 100% efficient solar cell? According to the principle of conservation of energy, the total energy of a closed system remains constant: energy can never be created or destroyed, but it can be transferred from onу form to another. In the real world, there is always a share of input energy transformed into 65

other forms of energy like heat, light, sound etc. Useful energy output is lower than total energy input, so efficiency is below 100%. The job of all photovoltaic cells is to transfer electromagnetic energy into electrical, however a fraction of input energy is dissipated through heat, for example. This is not the only factor limiting the efficiency of a solar cell, however it accounts for about 47% of total efficiency losses. 66

All the factors limiting the efficiency of a solar cell can be divided into two categories: internal and external. Internal factors are the ones which depend on the structure and materials of a solar panel while external factors are dependent on weather, location and other aspects which cannot improve with new technologies. We have already chosen the location with the highest level of annual solar irradiation which ensures the best external factors’ conditions available. Therefore, the only factors we might improve in the future are internal ones. To understand the nature of internal factors we need to have an idea of how do PV panels work. How do solar panels work?

Vikram Aggarwal, ‘What are the most efficient solar panels on the market?’, Energy Sage, 62

<h?ps://news.energysage.com/what-are-the-most-efficient-solar-panels-on-the-market/> [accessed 29 September 2017]

Ibid.63

‘New world record for solar cell efficiency at 46% – French-German cooperaion confirms compeiive advantage of 64

European photovoltaic industry’, Fraunhofer InsUtute for Solar Energy Systems, <h?ps://www.ise.fraunhofer.de/en/press-media/press-releases/2014/new-world-record-for-solar-cell-efficiency-at-46-percent.html> [accessed 2 October 2017]

Graham Bone, Gurinder Chadha, Nigel Saunders, A Level Physics for OCR, ed. By Gurinder Chadha (Oxford: Oxford 65

University Press, 2015), p. 75

‘Solar Efficiency Limits’, Solar Cell Central, 66

<h?p://solarcellcentral.com/limits_page.html> [accessed 6 September 2017]

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Solar panels are made up of smaller units called solar cells or photovoltaic cells connected with each other. The most common solar cells are made from silicon, a material known as a semiconductor as it conducts electricity and it is the main material for solar cells. A silicon solar cell uses two different layers of silicon in contact with each other which are sandwiched between conductive layers. 67

Figure 18 – Structure of a silicon solar cell 68

An N-type silicon has extra negatively charged electrons which are free to move. A P-type silicon has extra spaces for electrons called holes which can also move freely but have a positive charge. Between the N-type and P-type silicon there is an area called the p-n junction. 69

At the p-n junction, some of the electrons and holes come together, forming an electric field, which acts as a barrier. The electric field repels the free electrons and holes and keeps them from moving between layers. 70

� Figure 19 – Electric field at the p-n junction 71

What is the role of light in the electricity generation process inside a PV cell? To answer this question, we need to understand the principle of a photovoltaic effect. PV effect is a process in which two dissimilar materials (N-type and P-type silicon) in close contact produce an electrical current when struck by light or other radiant energy. Light is the flow of tiny particles called 72

photons, shooting out from the Sun. When one of these photons strikes the silicon cell with

Richard Komp, ‘How do solar panels work?’, TED-Ed, 67

<h?ps://ed.ted.com/lessons/how-do-solar-panels-work-richard-komp> [accessed 3 May 2017]

‘How Solar Panels Work’, Save On Energy, 68

<h?ps://www.saveonenergy.com/how-solar-panels-work/> [accessed 17 August 2017]

Richard Komp, ‘How do solar panels work?’, TED-Ed69

‘How do Photovoltaic Panels Generate Electricity?’, Solar Facts, 70

<h?ps://www.solar-facts.com/panels/how-panels-work.php> [accessed 1 November 2017]

‘How Solar Panels Work’, Save On Energy71

Grace Young, ‘Photovoltaic effect’, Encyclopedia Britannica, 72

<h?ps://www.britannica.com/science/photovoltaic-effect> [accessed 2 November 2017]

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enough energy, it can disrupt the bonds in both P-type and N-type silicon, creating free moving electrons and holes in both layers. An existing electric field will force free electrons and holes to move back to their proper layers but will not allow to cross the p-n junction any more. The electrons will be drawn to the N-side and holes to the P-side. 73

Figure 20 – Electrons move through an external circuit 74

To get an electrical current from PV cells free electrons travel from one point to another doing work in between. The mobile electrons are collected by thin metal conductors at the top of the cell. If you connect both N-type and P-type layers with a circuit, attractive force will drive free electrons back to holes through this circuit, creating an electric current that is going into the electrical network of a house. Electrons are the only moving parts in a solar cell, and they all go back where they came from, so solar cells can last for decades. 75

Factors limiting solar cell efficiency

When a photon strikes a silicon, it can disrupt the bonds creating a free moving electron or a hole depending on the layer, however, a photon does so only when it has enough energy. Visible light is only part of the electromagnetic spectrum. Electromagnetic radiation is made up of a range of different wavelengths, and therefore energy levels. The higher the frequency or 76

the lower the wavelength of a photon, the more energy it has. Gamma-rays have the most energy while radio waves have the least.

Richard Komp, ‘How do solar panels work?’, TED-Ed73

‘How Solar Panels Work’, Save On Energy74

Richard Komp, ‘How do solar panels work?’, TED-Ed75

Jessika Toothman and Sco? Aldous, ‘How Solar Cells Work’, How Stuff Works, 76

<h?ps://science.howstuffworks.com/environmental/energy/solar-cell4.htm> [accessed 3 November 2017]

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Figure 21 – Electromagnetic spectrum 77

For an electron to create current, its energy level must be increased from its normal valence level when it is tightly bound to one atom, to its higher energy conduction level when it is free to move around. However, electrons cannot accumulate energy from multiple photons, only one-78

to-one interactions are possible between photons and electrons. A photon can alter an 79

electron-hole pair if it has at least a certain amount of energy, measured in electron volts (eV) and defined by the cell material, the band gap energy of a material. 80

Bang gap energy = e * V = h * f = h * c / λ 81

V – Voltage output (V) e – elementary charge, the magnitude of the electric charge carried by a single electron (C) f – frequency of a photon (Hz) λ – wavelength of a photon (m) h, c – constants

If a photon’s energy is lower than the band gap energy (lower frequency / longer wavelength), it will simply pass through the cell as if it was transparent, so its energy will not be collected by a solar cell at all. This effect accounts for 18% of efficiency losses. On the other hand, if a 82

photon has more energy than the required amount (higher frequency / shorter wavelength), solar cell absorbs only the band gap energy while the extra energy gets dissipated as heat. However, X-rays and Gamma rays have just too much energy to be absorbed at all so all the energy of these spectrums is lost as heat. Overall, transformation of electromagnetic energy into heat accounts for 47% of total efficiency losses of a solar cell. The rest of efficiency 83

reduction is caused by the reflection of sunlight from the surface of the cell, losses at the junction of a silicon cell and a conductive material and manufacturing impurities in the silicon. Does it mean that to maximize the power output of a cell, the band gap of a material should be as low as possible? Power output is the product of voltage and current. If a material has a low band gap energy the number of electrons released per second increases since more photons have enough energy to free an electron, therefore, current increases. However, low band gap materials also have low voltage output since solar cell absorbs low energy photons. As a result, the optimal band gap, balancing these two effects, is around 1.34 eV for a cell made from a single material. 84

‘Electromagneic Radiaion’, Libretexts – Chemistry, 77

<h?ps://chem.libretexts.org/Core/Physical_and_Theoreical_Chemistry/Spectroscopy/Fundamentals_of_Spectroscopy/Electromagneic_Radiaion> [accessed 27 October 2017]

‘Solar Efficiency Limits’, Solar Cell Central78

Graham Bone, Gurinder Chadha, Nigel Saunders, A Level Physics for OCR, p. 24979

Jessika Toothman and Sco? Aldous, ‘How Solar Cells Work’, How Stuff Works80

Graham Bone, Gurinder Chadha, Nigel Saunders, A Level Physics for OCR, p. 24281

‘Solar Efficiency Limits’, Solar Cell Central82

Ibid.83

Jessika Toothman and Sco? Aldous, ‘How Solar Cells Work’, How Stuff Works84

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Figure 22 – Band gap and efficiencies 85

The entire available spectrum of sunlight, from infrared to ultraviolet, covers a range of about 0.5 eV to 2.9 eV. Therefore, a material with a band gap between 1.0 and 1.7 eV makes an 86

effective solar semiconductor. In this range, electrons can be freed without creating too much heat.

What is the maximum theoretical efficiency of a solar cell?

The maximum theoretical efficiency of a solar cell is called the Shockley–Queisser limit, first calculated by William Shockley and Hans-Joachim Queisser in 1961. The limit places 87

maximum solar conversion efficiency at around 33.7% under standard test conditions. It can 88

be obtained using a semiconductor with a band gap energy of 1.34 eV and a single p-n junction, meaning only one pair of N-type and P-type layers are used in a solar cell.

Figure 23 – Electromagnetic spectrum energy distribution 89

Electromagnetic spectrum energy distribution diagram shows why only a third of total energy can be absorbed by a crystalline silicon cell with a single p-n junction. The red colored area represents some infrared, all microwave, and all radio waves which do not have enough energy to be absorbed by a cell. The yellow area includes some infrared, visible light and some ultraviolet spectrums with too much energy, so the excess gets dissipated as heat. X-rays and

‘Solar Efficiency Limits’, Solar Cell Central85

Ibid.86

‘Shockley–Queisser limit’, Wikipedia, 87

<h?ps://en.wikipedia.org/wiki/Shockley%E2%80%93Queisser_limit> [accessed 17 August 2017]

Ibid.88

‘Solar Efficiency Limits’, Solar Cell Central89

� 20

Gamma rays are not providing any energy at all since they cannot be absorbed by a solar cell. Only the mustard colored area of radiation can be absorbed to create electricity in a crystalline silicon cell. Basically, the mustard area represents the Shockley-Queisser limit applied to silicon which shows that about 32% of available energy can be converted by a crystalline silicon cell 90

with a single p-n junction.

What will be the efficiency of solar panels in the future?

The Shockley–Queisser limit only applies to cells with a single p-n junction, however cells with multiple layers can outperform this limit. If we use several materials with different band gaps in 91

one solar cell, we can achieve much higher efficiencies. Materials with high band gap can absorb high energy photons and skip low energy ones which can be collected by lower band gap materials. It is important that semiconductors with high band gap are placed on the top of the cell since these materials are ‘transparent’ for low energy photons which can be absorbed by low band gap semiconductors on the bottom of the cell.

Figure 24 – Electromagnetic energy absorption by a three-layer cell 92

As we already know, a single layer PV cell can reach a maximum theoretical efficiency of 33.7%. The efficiency of a double layer cell can reach up to 42% and of a triple layer one up to 49%. In practice, hardly more than three junctions with different energy gaps are currently 93

used in solar cells, which enables efficiencies of about 40% to be achieved. The record for a 94

multi-junction cell is held by the Fraunhofer Institute for Solar Energy Systems at 46% using a four-junction cell approach. In the extreme, with an infinite number of layers, the efficiency limit 95

tends to 86.8% using concentrated sunlight. 96

‘Shockley–Queisser limit’, Wikipedia90

Ibid.91

‘Muli-juncion solar cell’, Wikipedia, 92

<h?ps://en.wikipedia.org/wiki/Muli-juncion_solar_cell> [accessed 29 September 2017]

‘Solar Efficiency Limits’, Solar Cell Central93

Alexey A. Toropov, Taiana V. Shubina, Plasmonic Effects in Metal-Semiconductor Nanostructures (Oxford: Oxford University 94

Press, 2015), p. 302

‘New world record for solar cell efficiency at 46% – French-German cooperaion confirms compeiive advantage of 95

European photovoltaic industry’, Fraunhofer InsUtute for Solar Energy Systems

‘Shockley–Queisser limit’, Wikipedia96

� 21

Obviously, we do not have an infinite number of semiconductors with different band gaps. Up to now, we have discovered almost 100 semiconductor materials, however, we cannot use all of 97

them for a PV cell production. Only materials with a band gap between 0.5 eV to 2.9 eV are suitable since this range covers the entire available spectrum of sunlight, from infrared to ultraviolet. How many semiconductor materials are there with a suitable band gap then? If we include even those which are very rare and expensive there should be about 36 materials with 98

a band gap fitting into the required range. If we used all 36 materials in a single PV cell, the maximum theoretical efficiency would be 72%. 99

Note that 72% is an approximation of what might be the efficiency of a solar panel in the future. These solar cells have never been created yet, so we cannot be absolutely sure in the accuracy of the result. To achieve such a high level of efficiency several assumptions should be made. Firstly, layers of semiconductor materials must be extremely thin to allow photons to pass through up to 35 layers. In addition, an upper surface of a panel should be covered with a perfect anti-reflective coating to make sure that no light which can be potentially absorbed is reflected. Finally, we need to use perfectly transparent conductors to eliminate energy losses from conductors’ shadow cast on lower layers. These assumptions and the fact that 36 semiconductor materials should be used make manufacturing process of 72% efficient PV panels very complicated and, therefore expensive. However, technology development does not stand still so all the manufacturing challenges we face today might be easily overcome in the future.

‘List of semiconductor materials’, Wikipedia, 97

<h?ps://en.wikipedia.org/wiki/List_of_semiconductor_materials> [accessed 7 November 2017]

Ibid.98

Alexey A. Toropov, Taiana V. Shubina, Plasmonic Effects in Metal-Semiconductor Nanostructures, p. 30299

� 22

What area of the solar plant can we expect in the future?

With current technologies it is probably impossible to build a solar cell with 36 layers, however let’s assume that in the future it will not be a problem. What if we could build as many 72% efficient solar panels as we need? What would be the area of a solar plant fully packed with highly efficient solar panels and located in the most insolated part of the UK, which could supply the whole county with enough electricity? The answer is 1,950 km2. However, this is not the final area because we also need to consider module inter-row spacing to make sure that the shade from one row does not fall on the next one.

Figure 25 – Solar shading calculator 100

In the Southern England, the inter-row spacing should be at least 4 m. Let’s say that the dimensions of one PV panel are 2 m * 1 m, so its surface area is 2 m2. Since the panels are tilted, the whole length including a shadow cast and a solar panel itself is 5.7 m. Thus, the total area required for one unit and inter-row spacing behind it is 5.7 m2. PV panels would occupy only about a third of the total area, therefore about two thirds of area would be unused and shaded for the most time.

Nowadays, enormous 28,000 km2 would stay in shade and 15,000 km2 would be taken by solar panels themselves resulting to a total area of 43,000 km2. In the future, only 3,610 km2 will be shaded and solar panels will cover additional 1,950 km2 summing up to 5,560 km2 for a complete solar plant. If my predictions are accurate, in the future, we might manage to reduce the total area in almost 8 times. As a result, 5,560 km2 is theoretically minimum total area of a solar plant which could supply the UK with energy.

‘Solar Shading Calc’, RBI Solar, 100

<h?p://www.rbisolar.com/solar-shading-calc/> [accessed 10 November 2017]

� 23

A = E / (r * GTI * PR) = 1772.5 * 109 kWh / (0.72 * 1330 kWh/m2 * 0.95) = 1.95 * 109 m2 = 1,950 km2

2 m2 / 5.7 m2 = 0.35

15,000 km2 / 0.35 = 43,000 km2

43,000 km2 - 15,000 km2 = 28,000 km2

1,950 km2 / 0.35 = 5,560 km2

43,000 km2 / 5,560 km2 = 7.7

5,560 km2 - 1,950 km2 = 3,610 km2

Is this project realistic?

Figure 26 - United Kingdom map with marked area 101

The first problem of this project being realistic is that 5,560 km2 is still a large area. It is an equivalent of 2.3% of the total UK area or of about a quarter (27%) of Wales. In addition, 5,560 km2 is an estimated area since it is not realistic to assume that we can set up all solar panels in one place. Firstly, we cannot cover the entire South coast of England with solar panels because there are people living there in cities like Brighton, Southampton, Bournemouth, and many other towns and villages. Secondly, the national grid losses would be significantly higher since all the energy should be delivered from the South coast throughout the whole UK territory including remote regions of Scotland and Northern Ireland. Finally, we assumed that GTI value is uniform across the whole chosen area while in real life it is not. GTI can also vary slightly from year to year.

Figure 27 (left) – General daily variation in insolation 102

Figure 28 (right) – General electricity demand over a day 103

Another problem is that solar panels can produce electricity only between sunrise and sunset. They generate the most electricity at noon since insolation is the highest at that time. However, electricity demand tends to be the highest in the morning and in the evening peaking at around 18:00. Thus, energy surplus gained during the day should be stored in the batteries and then released at peak times. This means that in addition to a solar plant we would also need to build

United Kingdom, Google Maps101

Deepak Pandey, ‘How does a solar panel provide constant power during a normal day?’, Quora, 102

<h?ps://www.quora.com/How-does-a-solar-panel-provide-constant-power-during-a-normal-day> [accessed 17 November 2017]

‘The Grid 2025 Challenge’, University of Glasgow, 103

<h?p://www.physics.gla.ac.uk/~shild/grid2025challenge/data.html> [accessed 18 November 2017]

� 24

5,560 km2 / 242,495 km2 * 100% = 2.3%

5,560 km2 / 20,779 km2 * 100% = 27%

a battery plant to make the whole system work. There might also be periods when energy demand is higher than supply, during the winter for example. This may result to periodic blackouts which will harm many industries and leave population with no access to energy. Ways to make the project more realistic We can reduce the area taken by the solar plant of the future by adding already existing renewable plants which produce 86 TWh (7,392 ktoe ) of final energy. That is almost 5% of total 104

final energy consumption so we can reduce the area of our solar plant by 5% then. Therefore, we would need 270 km2 less area, the equivalent of the city of Birmingham (267.8 km2) . However, 270 km2 is relatively insignificant cut in total area since we would still 105

need to have 5,290 km2 for our solar plant.

In addition, in the future we might expect increase in the efficiency of national grid, manufacturing and transport industries and domestic goods. For instance, the efficiency of an average electric is 70% while a normal gasoline one is only 16% efficient, over four times less. In the future, the efficiency of PV cells might exceed predicted 72% if a completely new 106

solar cell technology will be invented.

There is also no need to install all solar panels in one place. It is actually better to install them in different locations to minimize national grid losses. Places like roofs of buildings, stadiums, airports, and even surface of roads and sea can be covered with solar panels without having any other alternative infrastructure to be built there.

To minimize the number of batteries required to keep energy surplus for peak times and to make sure that there will always be enough energy stored to supply peak demand we can combine solar and wind power. Unlike solar panels, wind turbines can work at night which helps to balance energy demand. At night wind turbines can sustain low energy demand and charge the batteries at the same time. In the morning demand rises but batteries already have enough energy to sustain it. During the whole day solar panels and wind turbines work together to sustain the demand and charge the batteries to have a back up at the peak time in the evening.

‘Renewable sources of energy’, p. 154, GOV-UK, 104

<h?ps://www.gov.uk/government/uploads/system/uploads/a?achment_data/file/633782/Chapter_6.pdf> [accessed 5 August 2017]

‘Birmingham’, Wikipedia, 105

<h?ps://en.wikipedia.org/wiki/Birmingham#Geography> {accessed 5 December 2017]

Maury Markowitz, ‘Wells to wheels: electric car efficiency’, Energy MaZers, 106

<h?ps://ma?er2energy.wordpress.com/2013/02/22/wells-to-wheels-electric-car-efficiency/> [accessed 21 November 2017]

� 25

86 TWh / 1,772.5 TWh * 100% = 4.9%

5,560 km2 * 0.049 = 270 km2

5,560 km2 - 270 km2 = 5,290 km2

Figure 29 – Variation of electricity demand, solar and wind power supply over a year 107

Furthermore, energy demand is higher during the winter since more energy is needed for heating. However, solar panels harvest more energy during the summer when insolation is higher. Wind turbines can help to remove this disbalance too since they tend to have higher output during the winter when the winds are stronger.

Conclusion

Nowadays, a solar plant able to supply with energy the whole UK would cover about 43,000 km2. In the future, we might manage to reduce this area to 5,560 km2. This area can be diminished further if we also consider already existing renewable plants and potential rise in the efficiency of energy-consuming sectors and infrastructure. Therefore, the problem of vast size of the solar plant might be eliminated in the future, especially if solar panels will be installed in previously unused places like roofs.

However, the problem of balancing energy demand will still be relevant in the future. Solar cannot cope with volatility of electricity demand since its power output depends on insolation which we cannot control. Using batteries to backup energy surplus can help to balance this volatility, however it will still not guarantee that stored energy will always be enough to cover the energy gap between supply and demand. The alternative solution is combining solar and wind power and supporting it with a battery system. It can help to reduce the required number of batteries since the energy gap will be smaller and eliminate the risk of blackouts.

Neither today nor in the future single solar power does not seem to be capable of coping with UK energy demand. However, combination of solar and wind power with a backup system of batteries has a potential of being the best way to produce energy in the future.

‘The Grid 2025 Challenge’, University of Glasgow107

� 26

Appendix: Glossary • Primary energy consumption is the direct use at the source of crude energy, that is, energy

that has not been subjected to any conversion or transformation process. It can be 108

obtained by adding final energy consumption and energy losses incurred during conversion and transformation. 109

• Final energy consumption refers to energy consumed by final end users. Final energy 110

consumption is smaller than primary energy consumption since it does not include energy losses incurred during conversion and transformation.

• ktoe (thousand tonnes of oil equivalent) is a unit of energy defined as the amount of energy released by burning thousand tonnes of crude oil. 1 ktoe = 11,630 MWh. 111 112

• Solar photovoltaic systems (PV) is a power system which uses solar panels based on photovoltaic technology to generate electricity.

• Electronvolt (eV) is the amount of energy gained (or lost) by the charge of a single electron moving across an electric potential difference of 1 volt. Since electric charge of an electron 113

is extremely small, sometimes the energy is written in electronvolts which is a unit of energy equal to approximately 1.6×10−19 joules.

• Standard Test Conditions (STC) are defined as the solar irradiation of one kilowatt per square meter, a module temperature of 25 degrees Celsius and a solar irradiation angle of 45 degrees. STC ensure a relatively independent comparison and output evaluation of 114

different solar PV modules. 115

‘Primary Energy Consumpion’, Glossary of StaUsUcal Terms, 108

<h?ps://stats.oecd.org/glossary/detail.asp?ID=2112> [accessed 5 July 2017]

‘Energy Consumpion in the UK’, p. 4, Department of Business, Energy & Industrial Strategy109

Ibid.110

‘Tonne of oil equivalent’, Wikipedia, 111

<h?ps://en.wikipedia.org/wiki/Tonne_of_oil_equivalent> [accessed 8 July 2017]

‘Convert thousand tonnes of oil equivalent to megawa? hours’, Unit Juggler, 112

<h?ps://www.unitjuggler.com/convert-energy-from-ktoe-to-MWh.html> [accessed 8 July 2017]

‘Electronvolt’, Wikipedia, 113

<h?ps://en.wikipedia.org/wiki/Electronvolt> [accessed 30 July 2017]

‘Standard test condiions (STC) – Definiion, Glossary, Details’, Solar Mango, 114

<h?p://www.solarmango.com/solar-mango-dicionary/standard-test-condiions/> [accessed 3 September 2017]

Niclas, ‘Standard Test Condiions (STC): definiion and problems’, Sinovoltaics, 115

<h?p://sinovoltaics.com/learning-center/quality/standard-test-condiions-stc-definiion-and-problems/> [accessed 3 September 2017]

� 27

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